1. Field of the Invention
This invention relates generally to a method for shutting down a fuel cell system to ensure freeze start capability and, more particularly, to a method for shutting down a fuel cell system to ensure reliable freeze start capability at temperatures as low as −25° C.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated at the anode catalyst to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons at the cathode catalyst to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode electrodes, or catalyst layers, typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). Each MEA is usually sandwiched between two sheets of porous material, the gas diffusion layer (GDL), that protects the mechanical integrity of the membrane and also helps in uniform humidity distribution. MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include anode side and cathode side flow distributors, or flow fields, for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
The membrane within a fuel cell needs to have a certain water content so that the ionic resistance across the membrane is low enough to effectively conduct protons. Membrane humidification may come from the stack water by-product or external humidification. The flow of reactants through the flow channels of the stack has a drying effect on the cell membranes, most noticeably at an inlet of the reactant flow. However, the accumulation of water droplets within the flow channels could prevent reactants from flowing therethrough, and may cause the cell to fail because of low reactant gas flow, thus affecting stack stability. The accumulation of water in the reactant gas flow channels, as well as within the GDL, is particularly troublesome at low stack output loads.
As mentioned above, water is generated as a by-product of the stack operation. Therefore, the cathode exhaust gas from the stack will typically include water vapor and liquid water. It is known in the art to use a water vapor transfer (WVT) unit to capture some of the water in the cathode exhaust gas, and use the water to humidify the cathode input airflow. Water in the cathode exhaust gas at one side of the water transfer elements, such as membranes, is absorbed by the water transfer elements and transferred to the cathode air stream at the other side of the water transfer elements.
High frequency resistance (HFR) is a well-known property of fuel cells, and is closely related to the ohmic resistance, or membrane protonic resistance of the fuel cell membrane. Ohmic resistance is itself a function of the degree of fuel cell membrane humidification. Therefore, by measuring the HFR of the fuel cell membranes of a fuel cell stack within a specific band of excitation current frequencies, the degree of humidification of the fuel cell membrane may be determined.
As discussed above, a membrane that is too wet may cause problems due to water accumulation within the gas flow channels and, during low temperature environments, freezing of the water in the fuel cell stack may produce ice that blocks flow channels thereby affecting system restarts. When a fuel cell system starts with a frozen stack, the stack generates internal waste heat and begins to warm up. The stack also generates water during the warm-up process. There is generally a race between the stack heating up and the stack generating water. If too much water is made before enough heat is generated, the water may freeze in the stack and block gas flow, causing cell voltages to drop, which may cause the fuel cell system to shutdown.
If a fuel cell stack has too much water in it from the last system shutdown, the water generated during a long start-up may block gas flow channels. Typically, the colder the stack is at start-up the longer it takes to adequately heat up and the more likely that the water generated during start up will block the gas flow channels. Therefore, at very cold start-up temperatures, such as below −15° C., it takes longer for the fuel cell stack to heat up to 0° C. Consequently, the shutdown method becomes very critical for a successful restart of the fuel cell stack, particularly when the fuel cell stack temperature is −15° C. or colder.
A typical method for shutting down a fuel cell system is to purge the cathode side with high air flow, thereby pushing and evaporating water out of the fuel cell system. The cathode side air flow purge is typically done until the HFR reaches a specific level of dryness. The higher the HFR value, the drier the membranes, and the specific level of HFR typically used to dry the stack upon shutdown, which is approximately 225 mΩ·cm2, is insufficient. Therefore, there is a need in the art for a more robust shutdown strategy when temperature conditions are below freezing. Furthermore, there is a need to ensure that the fuel cells of the fuel cell stack are dried in a uniform way to minimize cell to cell humidity gradients as well as fuel cell stack inlet to fuel cell stack outlet humidity gradients.